1. Field of the Invention
The present invention relates to a micro-mirror element to be incorporated in e.g. an optical switching device which switches optical paths between a plurality of optical fibers by changing the direction of light reflection.
2. Description of the Related Art
In recent years, optical communications technology is utilized widely in a variety of fields. In the optical communications, optical fibers serve as a medium through which optical signals are passed. For switching the optical signal passing through a given optical fiber to another fiber, so-called optical switching devices are often used. In order to achieve high quality optical communications, the optical switching device needs to have such characteristics as high capacity, high speed and high reliability in switching action. In view of these, micro-mirror elements manufactured by utilizing a micro-machining technology are attracting attention as a switching element to be incorporated in the optical switching device. The micro-mirror elements enable the switching operation without converting optical signals into electric signals between the optical paths on the input side and the output side of the optical switching device. This is advantageous in achieving the characteristics mentioned above.
An example of the optical switching device using the micro-mirror element manufactured by utilizing the micro-machining technology is disclosed in, for example, International Publication WO 00/20899 and a thesis “Fully Provisioned 112×112 Micro-Mechanical Optical Crossconnect with 35.8 Tb/sec Demonstrated Capacity (Proc. 25th Optical Fiber Communication Conf. Baltimore. PD12(2000))”.
FIG. 10 outlines a common optical switching device 500. The optical switching device 500 includes a pair of micro-mirror arrays 501, 502, an input fiber array 503, an output fiber array 504, and a plurality of micro lenses 505, 506. The input fiber array 503 includes a predetermined number of input fibers 503a. The micro-mirror array 501 is provided with the same plurality of micro-mirror elements 501 a each corresponding to one of the input fibers 503a. Likewise, the output fiber array 504 includes a predetermined number of input fibers 504a. The micro-mirror array 502 is provided with the same plurality of micro-mirror elements 502a each corresponding to one of the output fibers 504a. Each of the micro-mirror elements 501a, 502a has a mirror surface for reflection of light. The orientation of the mirror surface is controllable. Each of the micro lenses 505 faces an end of a corresponding input fiber 503a. Likewise, each of the micro lenses 506 faces an end of a corresponding output fiber 504a. 
In transmitting optical signals, light L1 coming out of the output fibers 503a passes through one of the corresponding micro lenses 505, thereby becoming parallel to each other and proceeding to the micro-mirror array 501. The light L1 reflects on their corresponding micro-mirror elements 501a, to be directed toward the micro-mirror array 502. The mirror surfaces of the micro-mirror elements 501a are oriented, in advance, in predetermined directions so as to direct the light L1 to enter their micro-mirror elements 502a. Then, the light L1 is reflected on the micro-mirror elements 502a, and thereby being directed toward the output fiber array 504. The mirror surfaces of the micro-mirror elements 502a are oriented, in advance, in predetermined directions so as to direct the light L1 into their output fibers 504a. 
As described, according to the optical switching device 500, the light L1 coming out of the input fibers 503a reaches the desired output fibers 504a due to the deflection by the micro-mirror arrays 501, 502. A given input fiber 503a is connected to an output fiber 504a in a one-to-one relationship. With this arrangement, by appropriately changing the reflection angle of the micro-mirror elements 501a, 502a, switching can be performed and the light L1 can be directed into a selected output fiber 504a. 
In the optical switching device 500 described above, the number of fibers increases as the size of optical communications network increases, and accordingly the number of the micro-mirror elements or the number of micro-mirror surfaces also increases. The increase in the number of mirror surfaces leads to an increase in the amount of wiring necessary for driving the mirror surfaces, and thus, an increasing area need be provided for the wiring per micro-mirror array. If the mirror surfaces and the wiring are formed in the same substrate, a pitch or an interval between the mirror surfaces should be increased accordingly to the increase in the area formed with the wiring. This poses a problem of increased size of the substrate or the micro-mirror array. In addition, increase in the number of mirror surfaces itself tends to increase difficulty of forming the mirror surfaces and the wiring in the same substrate.
In order to solve these problems, a micro-mirror element is proposed, in which a pivotable mirror surface and a wiring pattern for driving the mirror surface are formed in separate substrates. These substrates are connected to each other by an electroconductive spacer. (See JP-A-2003-344785, for example.) According to such an arrangement, the wiring pattern for driving the mirror surface is formed in a separate substrate from the one for forming the mirror surfaces. Therefore, it is possible to overcome the problem of increased pitch between the mirror surfaces. Accordingly, it is possible to prevent the micro-mirror elements from becoming unduly large.
When the number of fibers increases due to growth of optical communications network, the number of possible combinations of one-to-one connection between the fibers will increase. As the combination number increases, highly accurate control of the mirror surface direction is required. To this end, it is necessary to properly detect the operating states of the respective mirror surfaces.